Yes, you can. You can find detailed information on how to do that in these support articles:
- Importing Customers from Xero
- Importing Customers from Salesforce
- Importing Customers from QuickBooks Desktop
- Importing Customers from QuickBooks Online
Note: When you import customers from one of these sources they don't have geolocation information associated with their site or billing contact addresses. For more information about this please read the article: Why do bulk imported customers not show on the map?
You can also import your customers from a CSV file. Please see the support article, Bulk import and update customer information, for more information.